The application relates to a method for rapid cooling of a hot isostatic press and a hot isostatic press.
Conventional hot isostatic pressing (HIP) or autoclave furnaces are used in many fields. For example, solid workpieces or molding compounds composed of powder can be compacted in a matrix under high pressure and at a high temperature to connect different materials or materials of the same type. Typically, the workpieces are placed in a furnace with a heating system and the furnace is enclosed by a high pressure container. During or after the heating operation, a complete isostatic compaction can take place by the pressure of a fluid, such as a liquid and/or inert gas (e.g., argon), on all sides until the workpieces are optimally compacted. This method can also be used for post-compact components, for example components made of ceramic materials such as for hip joint prostheses, for aluminum castings in the construction of cars or engines, for cylinder heads of passenger vehicle engines, or for precision castings made of titanium alloys (e.g., turbine blades). During a post-compaction operation under high pressure and at a high temperature, pores that evolved during the production process can be closed, existing faults can be connected, and the joining properties can be improved. Another field of application is the production of components that are composed of particulate materials and close to their final contour. Components made of particulate materials can be compacted and sintered.
Conventional HIP cycles can last a very long time—from several hours to several days. A sizable portion of the cycle costs are due to the tie-up of capital in the machine hour rate, especially the relatively long periods of cooling the operating temperature to a more reliable temperature at which the pressing system can be opened with less danger. The cooling cycles generally account for about one third of the cycle time and offer few to no benefits from a process engineering viewpoint. It is known that the cooling operation is an important factor for the material properties of the parts that are to be produced. Many materials require that a defined maximum cooling rate be observed to maintain the quality of the material. In addition, during the cooling operation the workpiece must be cooled uniformly—evenly throughout the volume—rather than non-uniformly with different temperature zones. When large components are produced, the internal stresses at different temperatures may lead to distortions, cracks with a corresponding notch effect, or to complete destruction. Such problems can occur even in the case of small parts that are generally deposited in a frame or on a shelf in the furnace.
Autoclaves that circulate hot gas with or without mechanical aids (e.g., a blower) are known in the art. When used without mechanical aids, an autoclave can perform natural convection and re-distribution of gas because of existing or promoted temperature differences (e.g., heating or cooling at the outer walls); as the cooler fluid flows downwards, the warmer fluid rises. With the use of guide elements, the fluid flow can be controlled to circulate more uniform heating or cooling in the autoclave. Conventional autoclaves typically use guide or convection shells that include an upper and a lower open tube. During the heating operation, heat sources in the furnace provide a flow as a function of the arrangement of the heat source. During the cooling operation, the cooled fluid flows downwards between the convection shell and the cooling outer wall and pushes the warmer fluid upwards past the workpieces in the interior of the shell. At the top cover of the HIP system, the flow coming from the bottom pushes the fluid in the direction of the outer regions causing the fluid to flow downward between the outer wall and the shell, maintaining a continuous cooling process.
One embodiment for rapid cooling of an HIP system is disclosed, for example, in published German patent application DE 38 33 337 A1. In the case of this solution, in order to start rapidly cooling, a gas circulation between the hot space inside the insulating hood and the cold space outside the insulating hood is produced by opening the circulation with valves in a bottom space. The upper top cover of the insulating hood exhibits continuously open boreholes through which the hot fluid can exit. One drawback with this embodiment is that very cold fluid flows back from the bottom space into the hot space and makes direct contact with the load of the furnace and/or the workpieces. Therefore, the hot space is filled with cold gas from the bottom to the top. This feature has the drawback that, on the one hand, a sudden cooling can occur with adjustable parameters that are too uncertain, and no uniform cooling rate over the entire charge space can be achieved. In the case of large components, the non-uniform cooling can cause distortion, cracks, or destruction.
WO 2003/070 402 A1 discloses a method for cooling a hot isostatic press and a hot isostatic press. According to this method, hot fluid leaves the load space, is mixed with a cool falling fluid outside the load space, and the mixed fluid is recycled again into the load space. The method itself is complicated in its targeted conditions and, furthermore, requires, in addition, a complicated construction of an associated hot isostatic press with many guiding regions. Disadvantageous also is that the re-introduced mixed fluid can flow back in an uncontrollable manner into the load space, where under some circumstances it can lead to varying cooling rates if the undercuts of the load or the support structures of the load prevent proper flow through the load space. Furthermore, the gas, which is cooled to a mixing temperature, is conveyed from the bottom into the load space, a feature that undeniably leads to a temperature gradient between the bottom end and the upper end of the load space. Therefore, a uniform cooling rate cannot be achieved.